In the prior art, polymers, particularly urethane rubber, have been used for a variety of applications in which it is desirable that the product have some electrical conductivity, either throughout the product itself or at least on the exposed surface of the product.
One example involves rollers used in many printers to help transport paper or carry toner in electrograhic printing. The rollers are often made of a polymer such as polyurethane or covered with a similar polymer to facilitate their paper-carrying or toner transfer ability. Different materials, including rubber, may be used in place of polymers for these applications, but because these polymers are much more durable than rubber, they are preferred. Most polymers do not conduct electricity, however, and static charges, which adversely effect the operation of the printer, can build up on the rollers. A similar problem exists if rollers of the same material are used for other purposes, such as carrying semiconductors as part of a semiconductor manufacturing process. Other end uses require conductive or semi-conductive parts as well.
As a result, there have been attempts make such polymer parts electrically conductive. In some cases, the part made from the polymer has been coated with an electrically conductive material. Unfortunately, these coatings have short life spans, and some are toxic. Another approach has been to disperse an electrically-conductive material in the polymer when the part is being fabricated. These electrically-conductive materials have included metal powders such as silver, copper, and nickel, and also materials such as carbon black, graphite, or other conductive polymers. However, the resulting products have several serious drawbacks. In the prior art, in order to make the polymer even semi-conductive, a large amount of conductive filler, e.g., metal powder like carbon black, had to be used, often as high as 10% to 40% of the overall mixture by weight. This degraded the mechanical and thermal properties of the resulting polymer part. Moreover, because of the size, and nature of the conductive particles, as well as the way in which the particles were mixed with the polymer, conductivity was not very great.
Another related problem is that it is very difficult, due to the relative size and weight of the added particles and the difficulty in dispersing them into the polymeric composition, to achieve a uniform distribution of the conductive material throughout the polymer. As a result of an uneven distribution, the electrical conductivity of the resulting product is not uniform, and the resulting product's mechanical and thermal properties suffer as well. As a result of all this, in general, products made from such compounds have been far less than satisfactory, and in fact, in some applications, become high maintenance items.
Finally, a related problem is that it is often desirable to select the specific conductivity of an polymer in advance, as different end applications preferentially require parts with different conductivities. Selection was not really possible with the prior art methods of making semi-conductive polymers.
Accordingly, one object of the invention is to provide a method by which polymeric materials may be made electrically conductive, without the need for conductive coatings or large amounts of conductive fillers.
Another object of the invention is to provide a method of making an electrically conductive polymeric material so that the resulting product has uniform electrical conductivity throughout.
Another object of the invention is to provide a method of making an electrically conductive polymeric material so that the relative electrical resistance of the resulting product may be varied with a high degree of accuracy.
Another object of the invention is to provide a method of making an electrically-conductive polymeric material whereby the mechanical and thermal properties of the resulting product are not degraded from what would be expected with a similar material which was non-conductive.
Another object of the invention is to provide a polymeric material which is electrically conductive and which may be molded and machined.
Another object of the invention is to provide an electrically-conductive polymeric material that is free of voids.
Another object of the invention is to provide an electrically-conductive polymeric material that when used on the surface of rollers, e.g., in a printer, inhibits build-up a static charge on the roller.
Another object of the invention is to provide a polymeric material with good thermal stability.